UCD-led NASA study captures gamma-ray flashes from tropical storms

Within the turmoil of a powerful tropical storm lies a naturally occurring event that produces the highest-energy light found on Earth.

Known as terrestrial gamma-ray flashes (TGFs), these bursts last for less than a millisecond and produce gamma rays with tens of millions of times the energy of visible light.

While estimates put the number of TGFs occurring each day at around 1,000, the phenomenon had not been studied in minute detail – until now.

In a new research paper published to the Journal of Geophysical Research: Atmospheres, a team of NASA scientists led by University College Dublin’s (UCD) Oliver Roberts analysed TGFs launched by the largest and strongest weather systems on the planet.

Based on recorded data obtained by the Fermi Gamma-ray Space Telescope, the study was able to look through more than 4,000 TGFs in the hope of finding out what causes them.

Before now, scientists had suspected that TGFs arose from the strong electric fields near the tops of thunderstorms, which, under certain conditions, can create an upward ‘avalanche’ of electrons at nearly the speed of light.

When the super-fast electrons race by air molecules, their paths become deflected slightly, causing the electrons to emit gamma rays.

Weaker storms produce more TGFs

“One result is a confirmation that storm intensity alone is not the key factor for producing TGFs,” said Roberts, who is now at NASA’s Marshall Space Flight Center in Huntsville, Alabama.

“We found a few TGFs were made in the outer rain bands of major storms, hundreds of kilometres from the powerful eye walls at their centres, and one weak system that fired off several TGFs in a day.”

Throughout the study, the international team of researchers observed 37 TGFs associated with bad tropical storms, particularly Hurricane Julio in 2014.

“In our study, Julio holds the record for TGFs, firing off four within 100 minutes on 3 August 2014, another the day after, and then no more for the life of the storm,” Roberts said.

“Most of this activity occurred as Julio underwent rapid intensification into a tropical depression, but long before it had even become a named storm.”

This shows that even weaker tropical storms are capable of producing TGFs; in fact, they are more likely to create greater numbers of them, particularly as storm systems intensify.

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